Buggy code:
public abstract class PackagePart implements RelationshipSource { protected OPCPackage _container ; protected PackagePartName _partName ; protected ContentType _contentType ; private boolean _isRelationshipPart ; private boolean _isDeleted ; private PackageRelationshipCollection _relationships ; protected PackagePart ( OPCPackage pack , PackagePartName partName , ContentType contentType ) throws InvalidFormatException { } protected PackagePart ( OPCPackage pack , PackagePartName partName , ContentType contentType , boolean loadRelationships ) throws InvalidFormatException { } public PackagePart ( OPCPackage pack , PackagePartName partName , String contentType ) throws InvalidFormatException { } public PackageRelationship addExternalRelationship ( String target , String relationshipType ) { } public PackageRelationship addExternalRelationship ( String target , String relationshipType , String id ) { if ( target == null ) { <START_BUG> throw new IllegalArgumentException ( "target" ) ; <END_BUG> } if ( relationshipType == null ) { throw new IllegalArgumentException ( "relationshipType" ) ; } if ( ( _relationships ) == null ) { _relationships = new PackageRelationshipCollection ( ) ; } URI targetURI ; try { targetURI = new URI ( target ) ; } catch ( URISyntaxException e ) { throw new IllegalArgumentException ( ( "Invalid<seq2seq4repair_space>target<seq2seq4repair_space>-<seq2seq4repair_space>" + e ) ) ; } return _relationships . addRelationship ( targetURI , EXTERNAL , relationshipType , id ) ; } public PackageRelationship addRelationship ( PackagePartName targetPartName , TargetMode targetMode , String relationshipType ) { } public PackageRelationship addRelationship ( PackagePartName targetPartName , TargetMode targetMode , String relationshipType , String id ) { } public PackageRelationship addRelationship ( URI targetURI , TargetMode targetMode , String relationshipType ) { } public PackageRelationship addRelationship ( URI targetURI , TargetMode targetMode , String relationshipType , String id ) { } public void clearRelationships ( ) { } public void removeRelationship ( String id ) { } public PackageRelationshipCollection getRelationships ( ) throws InvalidFormatException { } public PackageRelationship getRelationship ( String id ) { } public PackageRelationshipCollection getRelationshipsByType ( String relationshipType ) throws InvalidFormatException { } private PackageRelationshipCollection getRelationshipsCore ( String filter ) throws InvalidFormatException { } public boolean hasRelationships ( ) { } public boolean isRelationshipExists ( PackageRelationship rel ) { } public PackagePart getRelatedPart ( PackageRelationship rel ) throws InvalidFormatException { } public InputStream getInputStream ( ) throws IOException { } public OutputStream getOutputStream ( ) { } private void throwExceptionIfRelationship ( ) throws InvalidOperationException { } private void loadRelationships ( ) throws InvalidFormatException { } public PackagePartName getPartName ( ) { } public String getContentType ( ) { } public ContentType getContentTypeDetails ( ) { } public void setContentType ( String contentType ) throws InvalidFormatException { } public OPCPackage getPackage ( ) { } public boolean isRelationshipPart ( ) { } public boolean isDeleted ( ) { } public void setDeleted ( boolean isDeleted ) { } public long getSize ( ) { } @ Override public String toString ( ) { } protected abstract InputStream getInputStreamImpl ( ) throws IOException { } protected abstract OutputStream getOutputStreamImpl ( ) { } public abstract boolean save ( OutputStream zos ) throws OpenXML4JException { } public abstract boolean load ( InputStream ios ) throws InvalidFormatException { } public abstract void close ( ) { } public abstract void flush ( ) { } public void clear ( ) { } }

Fix:
throw new IllegalArgumentException ( ( "target<seq2seq4repair_space>is<seq2seq4repair_space>null<seq2seq4repair_space>for<seq2seq4repair_space>type<seq2seq4repair_space>" + relationshipType ) ) ;